1. Field of the Invention
The present invention relates to an on-site type fluorine gas generator.
2. Description of the Prior Art
Fluorine gas is one of the key gases essential in the field of semiconductor production, for instance. While it is used as such in certain instances, the demand for nitrogen trifluoride gas (hereinafter referred to as “NF3 gas”) and like gases synthesized based on fluorine gas and intended for use as cleaning gases or dry etching gases in semiconductor manufacturing apparatus has been rapidly increasing. Further, neon fluoride gas (hereinafter referred to as “NeF gas”), argon fluoride gas (hereinafter referred to as “ArF gas”), krypton fluoride gas (hereinafter referred to as “KrF gas”) and the like are excimer laser oscillation gases used in patterning of integrated semiconductor circuits, and the raw materials thereof used in many cases are mixed gases composed of a rare gas and gaseous fluorine.
The fluorine gas or NF3 gas for use in the manufacture of semiconductors and the like is required to be highly pure with the impurity content as low as possible. On the sites of semiconductor manufacture, for instance, necessary amounts of fluorine gas are taken out of gas cylinders filled with nitrogen gas. It thus becomes very important to secure sites for storing such cylinders, store the gas safely, maintain the purity of the gas, and manage for such purposes. As for NF3 gas, for which the demand has been increasing lately, the demand tends to exceed the supply, hence there arises a problem that certain amounts of the gas should be in stock. In view of these, to have a fluorine gas generator or producer of the on-demand and on-site type at the site of use thereof is preferred to handling high-pressure fluorine gas cylinders.
Conventionally, fluorine gas is produced in a electrolytic cell such as shown in FIG. 3. The electrolytic cell body 201 is generally made of Ni, Monel, carbon steel or the like. At the bottom of the electrolytic cell body 201, a bottom plate 212 made of polytetrafluoroethylene or the like is disposed for preventing the hydrogen gas and fluorine gas generated from being mixed with each other. The electrolytic cell body 201 is filled with an electrolytic bath 202, namely a potassium fluoride-hydrogen fluoride system (hereinafter referred to as “KF-HF system”) in the form of a mixed molten salt. The cell or bath is divided into an anode chamber 210 and a cathode chamber 211 by means of a skirt 209 made of Monel or the like. Upon applying a voltage between a carbon or nickel (hereinafter referred to as “Ni”) anode 203 contained in the anode chamber and a Ni cathode 204 contained in the cathode chamber 211, electrolysis occurs and fluorine gas is produced. The fluorine gas generated is discharged through a product line 208, and the hydrogen gas formed on the cathode side is discharged through a hydrogen gas discharge line 207. There is a problem, however. Contamination by carbon tetrafluoride (hereinafter referred to as “CF4 gas”) generated simultaneously upon electrolysis and hydrogen fluoride gas (hereinafter referred to as “HF gas”) evaporating from the electrolytic bath, among others, makes it difficult to obtain highly pure fluorine gas.
For on-demand and on-site operation, automatic control of the electrolytic bath level in the electrolytic cell body 201 is indispensable to the safety in automatic operation. As regards the technology of controlling the fluctuation in electrolyte level, for instance, Laid-open Japanese Patent Application (JP Kohyo) H09-505853 corresponding to FP0728228B1, EP0852267B1, EP0965661B1 and U.S. Pat. No. 5,688,384 proposes the so-called start/stop (on/off) control. However, when electrolysis is carried out using this technology, there arises a problem. Namely, the electrolysis is interrupted upon occurrence of a certain extent of fluctuation in liquid level, and the electrolysis cannot be restarted until the electrolyte level returns to the original level.
Accordingly, it is an object of the present invention to provide a fluorine gas generator capable of generating highly pure fluorine gas stably and safely.